1. Field of the Invention
The present invention relates to an optical glass and a lens suitable for e.g. an object lens to be used for recording on or reading out from an optical recording medium such as CD, CD-R, CD-RW, DVD or MO.
2. Discussion of Background
Recording on an optical recording medium such as CD is carried out by condensing a laser beam by an object lens. It is common to employ a laser beam having a wavelength of 780 nm and an object lens having a numerical aperture of from 0.4 to 0.45. In a case where a higher recording density is desired, it is common to employ a laser beam having a wavelength of 650 nm and an object lens having a numerical aperture of 0.65.
In recent years, in order to further increase the recording density, it has been proposed to use a laser beam having a wavelength of from 400 to 415 nm, e.g. 405 nm and an object lens having a numerical aperture of from about 0.65 to 0.85. In order to increase the numerical aperture, the curvature or the refractive index of the object lens may be made large. However, if the curvature of the object lens is increased more than ever, there has been a problem that the thickness of the object lens will be so thick that the operation distance i.e. the distance between the object lens and the disk, cannot be secured. Accordingly, it is desired to increase the refractive index of the object lens.
As an optical glass having a large refractive index, which can be used as an object lens having such a large numerical aperture, a TeO2 type glass (hereinafter referred to as conventional glass) comprising 6 mass % of Na, 4 mass % of K, 12 mass % of Ba, 78 mass % of Te, and the rest being O (oxygen) is disclosed (JP-A-2001-180971). Further, the composition of the conventional glass represented by mol % comprises, based on the following oxides, 14.8% of Na2O, 5.8% of K2O, 9.9% of BaO and 69.4% of TeO2.
However, the object lens is required to have not only a large numerical aperture but also a large transmittance and high chemical durability. On the other hand, the above-mentioned conventional glass contains alkali metal oxides in a total amount of 20.6 mol % (=14.8+5.8), whereby it is possible that there will be a problem in chemical durability.
It is an object of the present invention to provide an optical glass and a lens which solve such problems.